In precision engineering, electron beam welding is used to weld together two or more elements of a weld target assembly into a single body. However, depending on the nature and structure of the elements, it may be necessary to perform different types of weld operation on respective areas of the elements forming the assembly.
For example, in electron beam welding, a penetrative weld is a type of weld operation for creating the functional weld join between respective elements of the assembly; i.e. for creating the strong weld join between the respective elements of the assembly which ensures the integrity of the final assembly is maintained during its use.
However, electron beam welding is also used to perform cosmetic weld operations to produce a cosmetically welded join. A cosmetic weld join is a join which typically does not contribute in any significant way to the overall strength of the welded join, but which may be used for other purposes instead, e.g. to improve the finish of the penetrative weld.
For the same weld target, a penetrative weld is a weld operation resulting in a deeper weld join than a cosmetic weld.
Weld penetration is defined simply by how far into the parent metal the molten metal exists during the weld process.
The common understanding is that a ‘penetrative weld’ is a weld where the molten metal, and hence the structural joining capability of the weld, is formed to a depth that is a substantial proportion of the parent metal. In the present application, the meaning of penetrative weld may be taken to mean that the molten metal has existed for the full depth of the joint.
However, a cosmetic weld is a weld that is performed not to improve the structural characteristics of the joint, but to improve the visual appearance of the joint. Often the reverse side of a penetrative weld can be of a rough appearance, or have small ridges or troughs. Where electron beam welding is concerned, a cosmetic weld is characterised by a low powered, unfocussed electron beam that serves to melt just the surface layer of the metal (i.e. there is no significant penetration of the molten metal into the parent metal). Surface tension in the molten metal then causes the metal to flow to form a smooth surface.
As well as being visually more attractive, this is also a benefit during handling of the resultant welded body (e.g. it is less likely to snag on or cut fingers) and it also reduces the likelihood of crack initiation under cyclic loading when the resultant welded body is in use. This is particularly important in relation to the present invention, which is particularly useful in the manufacturing of OGVs.
A typical weld process might include a penetrative weld performed on one side of a join, e.g. from the front, and a cosmetic weld performed over the penetratively welded region, or performed on the reverse side of the join, e.g. from the back.
Typically, in order for the cosmetic weld to be performed, e.g. on the reverse side, the weld target assembly is moved from a first position in which the penetrative weld is performed to a second position in which the cosmetic weld is performed. However, conventional welding systems or apparatuses are often not configured such that penetrative weld operations can only be performed on weld target assemblies in the first position or such that cosmetic weld operations can only be performed on weld target assemblies in the second position. This is because, for example, different weld target assemblies may require respectively different weld processes to be performed in (at least) the first and second positions and thus the weld systems and apparatuses are often configured accordingly.
For certain weld target assemblies, the penetrative weld operation must be performed only on a specific side of the assembly. This might be, for example, because performing the penetrative weld operation on the wrong side of the join would spoil or damage the assembly and render it unfit for purpose.
An example of a weld target assembly where the penetrative weld operation must be performed only on the correct side is an outlet guide vane (OGV) for use in a gas turbine engine. A typical OGV weld target assembly, to be subject to e.g. electron beam welding, comprises a blade portion and a foot member (defining a sub-assembly) and one or more weld tabs.
Weld tabs are known generally, and are used in e.g. electron beam welding to improve the quality of the final weld. Weld tabs are also referred to as run-off tabs or weld run-off tabs. In essence, weld tabs provide additional welding regions in which the weld beam can start and/or stop, so that any weld beam cavities created at the beginning and/or end of the weld operation are not present in the final welded assembly, but are present only in the weld tabs. Typically, the weld tabs are machined off after the welding process is complete. In this way, the weld join in the final assembly is more uniform and does not suffer from weld beam cavities, which would otherwise weaken the strength of the weld join.
Nonetheless, if the penetrative weld to join the foot joint to the blade is performed on the incorrect side of the join, then the blade can be distorted, rendering the resulting final OGV assembly unfit for purpose.
Indeed, not all OGV assemblies require the penetrative weld to be performed from the same side, so there is scope for error to perform the penetrative weld operation on the wrong side of the join.
Often, after a penetrative weld has been performed, a cosmetic weld is performed over the penetratively welded region, e.g. the penetratively welded join. However, with conventional weld systems and apparatuses, after a cosmetic weld has been performed, it is often not possible to determine quickly and easily whether or not the penetrative weld was in fact performed because the cosmetic weld masks definitive evidence of the penetrative weld.
Such problems are particularly notable in relation to electron beam welding (but are not exclusive to electron beam welding) for the following reasons. Weld target assemblies subject to electron beam welding are held in a vacuum whilst the weld process takes place, and it is expensive and time consuming to generate the vacuum. Therefore, where the weld process requires multiple weld operations (e.g. a penetrative weld and one or more cosmetic welds), all the weld operations are typically carried out in sequence without breaking the vacuum.
Thus, there is a risk, when performing a series of weld operations including both a penetrative weld and one or more cosmetic welds, that the penetrative weld is performed on the wrong side of a join or not at all, thereby spoiling/damaging the final assembly and rendering it useless.